This invention relates generally to an electrophotographic printing machine, and more particularly concerns a half-tone screen for modulating a light image of an original document being reproduced by the printing machine.
In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level. The light image of the original document irradiates the charged portion of the photoconductive member dissipating selectively the charge thereon. This records an electrostatic latent image on the photoconductive member corresponding to the original document being reproduced. Generally, heat settable particles are employed to develop the latent image. These particles are transferred from the latent image to a sheet of support material, in image configuration. Heat is then applied to the particles permanently affixing them to the sheet of support material.
Multi-color electrophotographic printing is substantially the same as the process heretofore discussed. However, a plurality of cycles are employed, each cycle being arranged to reproduce a different color contained in the original document. This requires filtering the light image of the original document to record an electrostatic latent image corresponding to a single color thereof. Each of these latent images is developed with appropriately colored toner particles. The particles are then transferred to the sheet of support material, in superimposed registration with one another. This forms a multi-layered toner powder image on the sheet of support material. The multi-layered powder image is permanently affixed to the sheet of support material by the application of heat producing a permanent colored copy of the original document.
Heretofore, it has been difficult to produce copies having subtle variations of tone and color. In order to overcome this problem, a half-tone screen is frequently interposed into the optical system. This screen produces a screen pattern which modulates the electrostatic latent image recorded on the photoconductive surface to form tone gradations. These tone gradations are achieved by producing half-tone dots or lines of varying size. In the highlight zones, the dots or lines are small increasing in size throughout the intermediate shades until they merge together in the shadow regions. At the highlight end of the tonal scale, there will be complete whiteness, while, at the shadow end, there will be nearly solid blackness. Various patents describe the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,193,381; 3,776,633; and 3,809,555.
There are two general approaches for creating half-tone images. One of these techniques is termed multiplicative. A multiplicative system employs a screen interposed into the optical light path. Thus, a light image of the original document passes through the screen as it irradiates the charged portion of the photoconductive surface. The screen modulates or finely divides the light image to form a half-tone image. In this manner, a modulated electrostatic latent image is recorded on the photoconductive surface. Alternatively, the light image of the original document may remain unmodulated, as is the case for sequential screening. In this approach, a half-tone screen is illuminated independently prior to, or subsequent to, irradiation of the charged portion of the photoconductive member with the light image of the original document. Both the screen pattern and electrostatic latent image are recorded independently and in superimposed registration with one another to produce a modulated electrostatic latent image on the photoconductive surface. U.S. Pat. No. 3,540,806 issued to Starkweather in 1970 is an example of a teaching of sequential screening. As disclosed therein, a light image of the original document is projected onto the charged portion of the photoconductive member, and subsequently or prior thereto, a light image of the screen pattern is projected thereon. In this manner, the electrostatic latent image is modulated.
Different types of screens may be employed in either multiplicative or sequential screening. For example, U.S. Pat. No. 521,659 issued to Levy in 1894 discloses a screen having a grid or parallel, mutually othogonal opaque lines. Each axis contains one set of thick parallel lines and one set of thin parallel lines. The spacing between adjacent thick and thin lines appears to vary. Similarly, U.S. Pat. No. 725,252 issued to Jacobson in 1903 discloses a half-tone screen comprising a grid of parallel lines. Each axis of the grid contains one set of thick parallel lines and a set of thin parallel lines. The spacing between adjacent thick and thin lines appears to be substantially the same. U.S. Pat. No. 1,919,481 issued to Rowell in 1933 describes a screen having a plurality of closely spaced parallel lines arranged to form a thick opaque region and a plurality of lines spaced further apart to form thin opaque regions. U.S. Pat. No. 3,627,526 issued to Donald in 1971 discloses a half-tone screen having a plurality of thick lines with a plurality of thin lines being mutually othogonal thereto. Finally, U.S. Pat. No. 2,719,790 issued to Monroy in 1955 discloses a half-tone screen having a plurality of sets of lines having different spaces therebetween and being repeated at a constant frequency. This forms a plurality of rectangles having a high density line pattern and a low density line pattern. However, in all of the foregoing screens there does not appear to be any optimization to achieve high quality copies in electrophotographic printing.
Accordingly, it is a primary object of the present invention to improve the screen employed in an electrophotographic printing machine to achieve high quality copies.